Characterization of fast-growing foams in bottling processes by endoscopic imaging and convolutional neural networks

Regardless of whether the occurrence of foams in industrial processes is desirable or not, the knowledge about the characteristics of their formation and morphology is crucial. This study addresses the measuring of characteristics in foam and the trailing bubbly liquid that result from air bubble entrainment by a plunging jet in the environment of industry-like bottling process es of non-carbonated beverages. Typically encountered during the bottling of fruit juices, this process configuration is characterized by very fast filling speeds with high dynamic system parameter changes. Especially in multiphase systems with a sensitive disperse phase like gas bubbles, the task of its measurement turns out to be difficult. The aim of the study is to develop and employ an image processing capability in real geometries under realistic industrial conditions, e.g. as opposed to a narrow measurement chamber. Therefore, a typically sized test bottle was only slightly modified to adapt an endoscopic measurement technique and to acquire image data in a minimally invasive way. Two convolutional neural networks (CNNs) were employed to analyze irregular non-overlapping bubbles and circular overlapping bubbles. CNNs provide a robust object recognition for varying image qualities and therefore can cover a broad range of process conditions at the cost of a time-consuming training process. The obtained single bubble and population measurements allow approximation, correlation and interpretation of the bubble size and shape distributions within the foam and in the bubbly liquid. The classification of the measured foam morphologies and the influence of operating conditions are presented. The applicability to the described test case as an industrial multiphase process reveals high potential for a huge field of operations for particle size and shape measurement by the introduced method

The impact of the solutal Marangoni convection on the unsteady momentum and mass transfer at single droplets

Die Flüssig/flüssig-Extraktion ist eine häufig verwendete Grundoperation in der Chemieindustrie. Wichtige für die Auslegung benötigte Parameter werden auf Grundlage von Einzeltropfenexperimenten gewonnen. Besondere Bedeutung kommt der Phasengrenze zu, an der die relevanten Transportprozesse stattfinden. Verfügbare Modelle versagen oftmals für Systeme, in denen Marangonikonvektion auftritt. Diese kann den Stofftransport um ein Vielfaches verstärken und die Aufstiegsgeschwindigkeit temporär reduzieren. Marangonikonvektion ist bisher zu komplex für eine analytische Beschreibung. Der Einfluss der Marangonikonvektion auf Impuls- und Stofftransport von Einzeltropfen wird experimentell und numerisch untersucht. Aufstiegsgeschwindigkeit und Konzentration werden in einer Laborkolonne unter Variation von Anfangskonzentration, Stofftransportrichtung und Durchmesser als Funktion der Kontaktzeit ermittelt. Für die Simulationen wird der CFD-Code STAR-CD verwendet. Tropfen und Umgebung werden durch ein 360° 3D-Modell abgebildet, die Marangonikonvektion wird über die Schubspannungsbilanz implementiert. Der Tropfen ist kugelförmig und zu Beginn in Ruhe, Beschleunigungseffekte werden berücksichtigt. Ausgehend vom experimentell verwendeten Stoffsystem werden Parameterstudien durchgeführt. Marangonikonvektion verbessert die radiale Vermischung im Tropfen und verstärkt den Stofftransport im untersuchten Konzentrationsbereich um den Faktor 2-3 gegenüber dem System mit konstanter Grenzflächenspannung in beiden Transportrichtungen. Bereits nach der Tropfenbildung können bis ca. 60% des Stofftransports beendet sein. Das Beschleunigungsverhalten ist ein sensitiver Indikator für Stärke und Dauer der Marangonikonvektion, für die eine graphische Korrelation zur Vorhersage entwickelt werden kann. Formoszillationen verbessern den Stofftransport zusätzlich, der in diesem Regime mit einem modifizierten Modell nach Handlos & Baron beschrieben werden kann. Die Simulationen bestätigen qualitativ die fluiddynamischen Effekte. Der Stofftransport wird auch quantitativ gut beschrieben, im Bereich nicht-oszillierender Tropfen beträgt die Abweichung maximal 20%. Die Variation des Viskositätsverhältnisses zeigt im Gegensatz zur Vorhersage nach Sternling & Scriven immer Marangonikonvektion. Auch eine Reduktion des Grenzflächenspannungsgradienten um den Faktor 100 führt noch zu einer Verbesserung des Stofftransports. Wird dagegen der Verteilungskoeffizient erhöht, schwächt sich Marangonikonvektion ab. Bei Werten um 10 ist praktisch kein Einfluss auf den Stofftransport mehr erkennbar.Liquid/liquid extraction is a frequently used unit operation in chemical processes. Some important design parameters can be derived from single drop experiments. The relevant transport phenomena occur at the interface between droplets and the ambient liquid, the interfacial region is thus of particular importance. Available models oftentimes fail for systems where Marangoni convection occur. Marangoni convection can enhance mass transfer severalfold and is able to reduce the drop rise velocity temporarily. Up to now, Marangoni convection is too complex for an analytical description. The influence of Marangoni convection on momentum and mass transfer at single rising droplets is investigated experimentally and numerically. Drop rise velocity and mean concentration are determined under variation of initial solute concentration, mass transfer direction and droplet diameter as a function of contact time in a pilot plant. For the simulations, the CFD-Code STAR-CD is used. Droplet and ambient liquid are represented by a full 360° 3D model, the Marangoni convection is implemented via the shear stress balance. The droplet is spherical and initially at rest, acceleration effects are considered. Based on the experimental system, numerical parameter studies are carried out. Marangoni convection promotes radial mixing inside the droplet and enhances mass transfer in the investigated concentration range by a factor of 2-3 compared to the case with constant interfacial tension in both mass transfer directions. After droplet formation, up to 60% of mass transfer can already be completed. The acceleration behaviour is a sensitive indicator for strength and duration of Marangoni convection, a graphical correlation is derived to predict the reacceleration time. Shape oscillations can enhance mass transfer additionally, in this regime mass transfer is described with a modified Handlos & Baron model. The simulations confirm qualitatively the fluid dynamic effects. Mass transfer is also quantitatively in good agreement with the experiments. In the non-oscillating regime, the deviation does not exceed 20%. The variation of the viscosity ratio shows always Marangoni convection which is in contradiction to the Sternling & Scriven theory. Even a reduction of the interfacial tension gradient by a factor of 100 leads to significant mass transfer enhancement. But if the distribution coefficient is increased, Marangoni convection weakens. For values of the distribution coefficient around 10, practically no influence of Marangoni convection on mass transfer is noticeable

Marangoni instabilities in single droplet extraction systems

Single droplets immersed in an ambient liquid represent the smallest transfer unit in a liquid/liquid system. Dispersed multiphase extractive systems occur in many industrial processes. Conventional applications in chemical, oil or high temperature processes in metallurgical industries are more and more supplemented by applications in the food and pharmaceutical industries, biotechnology and environmental engineering. The transport phenomena in these polydispersed systems are linked in a highly complex manner via the properties of the droplet interface. Thus, the understanding of interfacial phenomena in extractive liquid/liquid systems is essential for the phenomenological and theoretical description of solute mass transfer across the interface of droplets. In particular the Marangoni convection has potential to substantially increase the interfacial molar flux, i.e. the mass transfer coefficient, and impacts in a highly complex manner the fluid dynamics of the system, provided the interface is free from contaminants and surfactants. However, in industrial applications, surfactants can hardly be avoided (or they are essential for the process). These surfactants can significantly affect the mobility of the interface, and on the other hand, due to their ability to lower locally the interfacial tension, they can initiate Marangoni effects and thus additional interfacial convections. The Marangoni convection occurs at interfaces with significant interfacial tension gradients, originated from differences in interfacial solute concentration, temperature or electrocapillary potential. The system tends to minimize its surface energy and expands regions of lower towards regions of higher interfacial tension. This process will generate a tangential interfacial convection which leads to - in cases of small damping - chaotic flow patterns within the droplet. These flow patterns promote inner radial mixing and thus transfer of matter. Whenever Marangoni convection occurs, there is a two-way coupling between concentration and flow field which makes the whole process complicated. Despite many research activities on this field, a complete physical understanding is still missing, and modelling attempts did not lead to satisfying and reliable results. In the work presented here, detailed experimental investigations of the impact of Marangoni convection on mass transfer and fluid dynamics at single droplets have been carried out in a well-proven test extraction column supported by the development of a fully 3D CFD model

Towards a slag droplet heat exchanger - capillary

Molten slag contains a considerable amount of sensible heat which can be recovered provided that a large specific surface area is created to facilitate heat transfer to an ambient gaseous medium. It is preferable to disperse the molten slag into uniformly sized droplets in order to permit a more reliable process design. The basic concept is to distribute a volume of molten material into coherent ligaments or jets which consecutively break into droplets by action of capillary forces. This can be done either radially using centrifugal forces as currently explored in the dry slag granulation process, or vertically by forming cylindrical liquid jets issuing from capillaries or nozzles as proposed in the direct contact droplet heat exchanger (DHX). The latter option is explored in this paper investigating the controlled breakup of molten calcia/alumina jets at 1660°C in a recently commissioned three-zone high temperature furnace

Novel High-Temperature Experimental Setup to Study Dynamic Surface Tension Phenomena in Oxide Melts

In many pyrometallurgical applications, subprocesses such as emulsification, droplet, bubble or jet formation, coalescence, and surfactant adsorption occur at small time scales (typically milliseconds to fractions of seconds), both at slag/metal and slag/gas interfaces. These phenomena are surface tension driven anddue to the high-temperature environmentvery difficult to investigate in a quantitative manner. Under these dynamic conditions, the instantaneous surface tension of slags may vary in time as well as along its surface and may change dramatically the rate of the involved processes. This paper presents a new high-temperature experimental setup to study and measure the dynamic surface tension of slags, the mechanisms of slag jet and droplet formation, and the capillary breakup of molten slag jets. It features a three zone furnace with optical access, and a droplet generation device incorporating a back-pressure system in combination with a stopper for precise slag flow control. The first successful results of controlled formation of calcia/alumina droplets and coherent jets in an argon environment are discussed. Various time-dependent phenomena such as droplet formation and elongation, necking, breakup, oscillation, satellite formation, and jet disintegration were investigated and quantified using a high-speed camera system. A dynamic pendant drop method was applied to determine the surface tension. The obtained values are in excellent agreement with literature data

Energy and Resource Savings through Innovative and CFD-based Design of Liquid/Liquid Gravity Separators

Computational-Fluid-Dynamics (CFD)-Simulationen in Kombination mit Tropfenpopulationsbilanzen führen zu einem praxisgerechten Standard, um auf Basis verfügbarer Prozessdaten den Strömungsverlauf – und damit die Verweilzeitverteilung – in liegenden Abscheidern beliebiger Größe zu berechnen. Durch Implementierung des Tropfenverhaltens wird auch die Berechnung eines tropfenspezifischen Abscheider-Wirkungsgrades ermöglicht. Die Methodenentwicklung erfolgte mit baugleichen Anlagen an drei verschiedenen Standorten. Die darauf beruhenden CFD-Simulationen wurden erfolgreich mit experimentellen Daten der beteiligten Industriepartner validiert.ERICA